DHCP for static IPs

Most LAN administrators use the DHCP protocol to dynamically assign IP addresses to "clients" which are usually workstations and laptops that enter and leave and move around on the network regularly. In practice it is much easier to assign a pool of addresses and let DHCP take care of it, than to keep track of all the static IP addresses manually.

Conversely, most LAN administrators insist on using static, manually assigned addresses to servers. Often, this is because of DNS (domain name to IP address mappings) and because of a perception that DHCP is a "point of failure". That is, that if DHCP becomes unavailable for some reason, that users would not be able to access the servers.

However, many LAN administrators may not be aware that DHCP can also be used to assign IP addresses that don't change. This feature is very useful when a specific user needs to keep their IP address for some reason. A good example of this is a firewall rule, where you only allow that IP address to access a certain site. Another example would be a number of scientific journals that are published on the Internet which use the source IP address of the user for licensing, so users must always have the same IP address.

DHCP for static IPs is also useful for most types of servers, and occasionally even for network equipment like routers. In the case of servers, this feature gives administrators a central place to control IP addressing. In volatile environments, it can also

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make it easy to move servers around or replace them. It can be used as a cheap way to fail a service over from one piece of hardware to another, as well.

The downside of this method is that you have to keep track of the MAC address of the server, which is how the feature operates. On the DHCP server, you will have to type in the MAC address and assign an IP address to it. From then on, that IP address will only be leased to requests that contain that specific MAC address.

Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.

This was first published in August 2004

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